1. The Field of the Invention
The present invention is related to radio transmission equipment, and more specifically to directional antenna systems featuring phase shifting.
2. Technical Background
AM and FM radio broadcasts are extremely popular for their transmission of audio programs. AM and FM radio has been employed for several decades and can be found world wide. Radio receivers have numerous embodiments including various portable receivers to be manually carried as well as being incorporated into vehicles. AM radio has a disadvantage in that it does not provide high fidelity as does FM radio. As such, AM radio has not enjoyed quite the popularity that FM radio has for musical broadcasts. AM radio is nevertheless widely used for talk show and news broadcasts.
In general, FM radio stations are valued more than AM radio stations because of the improved sound quality. Accordingly, popular music stations prefer to broadcast on FM frequencies and the AM frequency band is generally viewed as being inferior to the FM band.
Nevertheless, AM systems have advantages that are not found with FM systems. For example, AM signals can propagate further than FM signals. Directional AM systems also allow for directed power and different day and night time power levels and directional patterns to reduce interference. AM frequencies are also more available and less expensive than FM frequencies.
Conventional AM directional array systems include phase shifter components that employ a process of envelope delay and power division to create a desired directional pattern. Phase shifting components are typically reactive and are collectively referenced herein as a phasor. The phasor includes inductive and capacitive components to create the envelope delay and power ratio relative to a reference tower. The reactive components change the phase, relative to a reference tower, of the output signal flowing to one or more broadcast towers and thereby create the envelope delay. The reactive components also introduce distortions in the envelope. The reactive components of the phasor further divide the transmitter output power into proper ratios that are delivered to the towers.
Phasors have many limitations that make them difficult and expensive to use. The reactive components of the phasor are typically large and require mechanical systems that are difficult to adjust and configure. Phasors also are subject to drift caused by the environment, create signal distortions, and are maintenance intensive. Furthermore, as the power output and the number of towers in a directional array increases, the cost of a phasor increases exponentially. Phasors are relatively expensive to operate at high power and modulation levels, multiple power levels, and with multiple tower configurations. In numerous instances large, high power directional arrays are required to adequately serve the community. Phasors are typically custom built, adding to their expense.
An ideal phasor presents a load to the transmitter that has the same impedance at all side band frequencies. However, conventional phasors are comprised of inductors and capacitors, whose reactance varies with frequency. Thus, ideal conditions are difficult to achieve with a conventional phasor.
For proposed digital communication techniques, digital modulation envelopes are required. However, digital modulation envelopes may be affected by distortions caused by the reactive components within a phasor. Digital modulation systems typically require that a load be linear across the width of the band. Phasors, with their inherent instability, make achieving true load linearity difficult. Furthermore, phasors create envelope delays at high signal levels which are outside the digital domain. This precludes the use of digital components that may overcome the limitations of the reactive components.
It would be an advancement in the art to improve AM transmission systems by eliminating the need for expensive and difficult reactive components and create a phasor that is mass-producible. It would be a further advancement in the art to create the envelope delay necessary for directional operation at a low signal level within the digital domain and control the power delivered to each tower at the power amplifier stage. Such advancements would improve the operation of AM transmission systems and help increase the commercial viability of AM stations.